1. Field of the Invention
The present invention relates to an absorbent structure comprising an acidic or basic water-swellable, water-insoluble polymer, a basic or acidic material, and, optionally, a buffering agent, wherein the absorbent structure exhibits desirable absorbent properties. Specifically, the present invention relates to an absorbent structure having the ability to absorb a large quantity of liquid while maintaining a substantially desired and balanced pH profile on or along the upper surface of the absorbent structure. The absorbent structure is useful in disposable absorbent products, such as those disposable absorbent products that are used to absorb bodily liquids.
2. Description of the Related Art
The use of water-swellable, generally water-insoluble absorbent materials, commonly known as superabsorbents, in disposable absorbent products is known. Such absorbent materials are generally employed in disposable absorbent products such as diapers, training pants, adult incontinence products, and feminine care products in order to increase the absorbent capacity of such products while reducing their overall bulk. Such absorbent materials are generally present in disposable absorbent products in a fibrous matrix, such as a matrix of wood pulp fluff. A matrix of wood pulp fluff generally has an absorbent capacity of about 6 grams of liquid per gram of fluff. The superabsorbent materials generally have an absorbent capacity of at least about 10, preferably of about 20, and often of up to 100 times their weight in water. Clearly, incorporation of such absorbent materials in disposable absorbent products can reduce the overall bulk while increasing the absorbent capacity of such products.
The superabsorbent material commonly used in disposable absorbent products is the substantially neutralized form of a crosslinked polymer, such as the sodium salt of a crosslinked polyacrylic acid. The salt form of a crosslinked polymer is generally used since the capacity for the absorption of aqueous liquids of a crosslinked but substantially unneutralized polymer is typically very low as compared to the neutralized, or salt, form of the crosslinked polymer. However, one potential beneficial aspect of using the unneutralized form of a crosslinked polymer is that such a material has the capacity to exchange some of the cations present in urine and other body liquids that typically insult disposable absorbent products. In contrast, the substantially neutralized form of a crosslinked polymer generally does not allow for such ion-exchanging to occur.
It is, therefore, an object of the present invention to use the substantially unneutralized form of a crosslinked polymer in a disposable absorbent product in combination with another material which would neutralize the crosslinked polymer in situ when urine or other body liquids contact the disposable absorbent product. The use of the substantially unneutralized form of a crosslinked polymer would help reduce the ionic content of the body liquid through ion-exchange. The reduction in the ionic strength of a body liquid contacting a disposable absorbent product is generally beneficial in that the absorbent capacity of a crosslinked polymer is generally inversely related to the ionic strength of the liquid being absorbed. Furthermore, the synthesis of the substantially unneutralized form of a crosslinked polymer generally provides a better crosslinked polymeric network as compared to the synthesis of the substantially neutralized form of the crosslinked polymer, in that the formation of polymeric network defects is generally minimized which tends to increase the absorbent capacity of the crosslinked polymer. Therefore, another potential benefit of using the substantially unneutralized form of a crosslinked polymer, which is neutralized in situ, in a disposable absorbent product is the improvement in liquid absorption and distribution that occurs in the disposable absorbent product as problems caused by the rapid swelling of the substantially neutralized form of the crosslinked polymer can be avoided.
A complexity to the use of a substantially unneutralized form of a crosslinked polymer, with in situ neutralization of the crosslinked polymer, is the need to maintain a balanced pH profile on the surface of the disposable absorbent product. As the crosslinked polymer is being neutralized in situ after contact with a body liquid, temporary imbalances in pH can occur due to differences in the rate of dissolution or ionization of the material being used to neutralize the crosslinked polymer and the diffusion of the ionic species to the unneutralized sites in the crosslinked polymer which allows neutralization to occur. This temporary pH imbalance could result in an undesirable alkaline pH or an acidic pH in the disposable absorbent product next to a user's skin with the potential for skin irritation. As such, there is a need for controlling the pH in that portion of the disposable absorbent product that comes into contact with or is otherwise near or next to a wearer's or user's skin. By better controlling the pH in that portion of the disposable absorbent product that comes into contact with or is otherwise near or next to a user's skin, the incidence of skin irritation should be reduced.
It is, therefore, an object of the present invention to provide an absorbent structure that absorbs a large quantity of liquid, such as at about the same final capacity as compared to an absorbent structure comprising commercially available superabsorbent materials, wherein the absorbent structure maintains a substantially desired and balanced pH profile on or along the upper surface of the absorbent structure.
It is also an object of the present invention to provide an absorbent structure that comprises a substantially unneutralized form of a crosslinked polymer in combination with an inexpensive material that neutralizes the crosslinked polymer in situ since such an approach may reduce the overall cost of the absorbent structure.
It is also an object of the present invention to provide an absorbent structure that may be prepared simply and with a minimum of materials and additives so as to reduce the overall cost of preparing the absorbent structure as well as reduce the potential deleterious effect that such additives might have on the overall absorbent properties of the absorbent structure.
It is also an object of the present invention to provide an absorbent structure that exhibits unique properties so that such absorbent structure may be used in novel applications.